In recent years, color liquid crystal display devices have been widely used in various fields such as monitors for portable notebook personal computers, [desktop] personal computers, and the like, or as display parts for liquid crystal television sets, video-integrated type liquid crystal television sets, portable telephones, portable information terminals, and the like. Furthermore, with the increase in the amount of information processed, the diversification of needs, the need to handle multimedia, and the like, there has been an active pursuit of increased screen size and higher definition in liquid crystal display devices.
Liquid crystal display devices are basically constructed from a back lighting part and a liquid crystal display element part. In the back lighting part, from the standpoint of compactness of the liquid crystal display device, an edge lighting system in which the primary light source is disposed so as to face the side end surface of the light conductor has been widely used.
Incidentally, in recent years, in liquid crystal display devices and the like, a high-brightness light source device in which the utilization efficiency of the light is increased in order to achieve effective utilization of the quantity of light that is emitted from the primary light source has been utilized in portable notebook personal computers, portable telephones, portable information terminals, and the like as an edge lighting type back lighting part from the standpoint of reducing power consumption.
For example, light source devices in which a lens surface in which numerous lens units are formed, or a pearskin finished surface, is formed on at least the light emitting surface of the light conductor or the back surface, and a prism sheet is carried on the light emitting surface, have been proposed in Japanese Patent Application Laid-Open No. H2-17 (Patent Reference 1) and Japanese Patent Application Laid-Open No. H2-84618 (Patent Reference 2). However, in such light source devices, although a relatively high brightness is obtained, such devices are still unsatisfactory in terms of the uniformity of the brightness at the light emitting surface. Accordingly, for the purpose of achieving a uniform brightness at the light emitting surface, a light source device in which the average inclination angle of the lens surface or pearskin finished surface formed on the light conductor is set at 0.5 to 7.5° has been proposed in Japanese Patent Application Laid-Open No. H7-288552 (Patent Reference 3) and the like. In such a light source device, however, although the uniformity of the brightness is improved, and a relatively high brightness is obtained as described in Patent References 1 and 2 above, sufficiently satisfactory results are not obtained from the standpoint of achieving a high brightness.
Furthermore, the formation of the prism surfaces of a prism row constituting a prism sheet into a convex surface shape in order to heighten the focusing and directionality of light, and thus to concentrate and emit the light in a relatively narrow range, etc., when the light emitted from the light conductor is deflected using a prism sheet, has been proposed in Japanese Patent Publication No. H9-507584 (Patent Reference 4), Japanese Patent Application Laid-Open No. H9-105804 (Patent Reference 5), Japanese Patent Application Laid-Open No. H11-38209 (Patent Reference 6) and Japanese Patent Application Laid-Open No. 2000-35763 (Patent Reference 7). However, these prism sheets concentrate the emitted light in a single direction (substantially in the normal direction) as much as possible before emitting this light, and do not cause the emission of light with directionality in two or more different directions.
Moreover, the construction of the prism surfaces of the prism row constituting a prism sheet from a plurality of flat surfaces for the same purpose as in Patent References 4 through 7 mentioned above has been proposed in Japanese Patent Application Laid-Open No. S63-298902 (Patent Reference 8) and Japanese Patent Application Laid-Open No. 2002-197908 (Patent Reference 9). However, in these prism sheets as well, the emitted light is concentrated and emitted as much as possible in a single direction (substantially in the normal direction), and light cannot be emitted with directionality in two or more mutually different directions.
In recent years, there has been a demand for liquid crystal display devices that cause the emission of light with directionality in at least two mutually different directions, especially at least three mutually different directions, while promoting a reduction in power consumption. The vehicle-mounted display device of a car navigation system may be cited as an example of such a liquid crystal display device. Such a vehicle-mounted display device can be used not only as an image display for a car navigation system, but also as the image display for a television or the like. Accordingly, since there may also be cases in which passengers sitting in the rear seat view the display images in addition to the driver and the passenger sitting in the front passenger seat, a display with the brightness required to be seen by the driver, passenger in the front passenger seat and passengers in the rear passenger seat must be accomplished using the limited power supply obtained by a vehicle-mounted battery. This vehicle-mounted display device is generally disposed between the driver's seat and the front passenger seat so that the display screen is perpendicular to the direction of advance of the vehicle. Accordingly, a directionality in two directions forming an angle of approximately 25 to 35° on mutually opposite sides with respect to the normal direction of the display screen is required for the driver and the passenger in the front passenger seat, and a directionality substantially in the normal direction of the display screen is required for the passengers in the rear seat.
However, in the light source devices described in Patent References 4 through 9, the emitted light is emitted with a high directionality in only one specified direction (e.g., substantially in the normal direction of the display screen); accordingly, in other directions, the brightness shows an extreme drop, so that these devices have not been suitable for use as light source devices in vehicle-mounted display devices.
Furthermore, display devices for game machines or the like that can be played by or observed by at least two players or observers in at least two positions determined beforehand with respect to the display screen are indicated as examples of such display devices in which directionality in two or more directions is required.    Patent Reference 1: Japanese Patent Application Laid-Open No. H2-17    Patent Reference 2: Japanese Patent Application Laid-Open No. H2-84618    Patent Reference 3: Japanese Patent Application Laid-Open No. H7-288552    Patent Reference 4: Japanese Patent Application Publication No. H9-507584    Patent Reference 5: Japanese Patent Application Laid-Open No. H9-105804    Patent Reference 6: Japanese Patent Application Laid-Open No. H11-38209    Patent Reference 7: Japanese Patent Application Laid-Open No. 2000-35763    Patent Reference 8: Japanese Patent Application Laid-Open No. S63-298902    Patent Reference 9: Japanese Patent Application Laid-Open No. 2002-197908